1. Field of the Invention
This invention relates to a voltage-resonant DC-DC converter, and in particular to an improvement in the power-loss characteristic of the primary switch and the output-regulation characteristics of multiple output circuits provided in the converter.
2. Description of the Related Art
It is well known in the art that a turn-on loss in a DC-DC converter is markedly reduced by applying the zero-volt switching (ZVS) method to the voltage-resonant converter, and thus the converter of this type is now further developed in order to attain high efficiency at high frequency.
FIG. 1 represents a circuit of a typical single-ended voltage-resonant forward converter. The circuit is made up of DC power source 1, transformer 5, primary switch 3, resonance capacitor 4 connected in parallel with primary switch 3, voltage-clamping circuit 70, rectifier diode 11, current snubber circuit 71, input condenser 2, output condenser 12 and loop-control circuit 14 for controlling switching of primary switch 3. In the figure, inductor 15 represents a leakage inductance of transformer 5 and an inductance of an externally attached inductive element if required.
Both primary switch 3 and capacitor 4 are connected in series with primary winding 6 of transformer 5. Primary switch 3 driven under control of loop control circuit 14, controls the output of the converter. Capacitor 4, associated with a leakage inductance of primary winding 6, i.e. self-inductance L.sub.P of primary winding 6, constitutes a series resonance circuit, and provides primary switch 3 with zero voltage on turning-on of the switch (the E-class switching mode). In this way power loss produced on turning-on primary switch 3 (a turn-on power loss) is reduced.
Voltage clamping circuit 70 is provided for resetting a magnetic energy stored in the magnetic core of transformer 5 during the turn-on period, in order to prevent the core from being magnetically saturated. The circuit is made up of a diode, a resistor and a capacitor interconnected in parallel, and connected in parallel with primary winding 6. The magnetic energy stored in the core of transformer 5 is dissipated by the resistor. The capacitor in the circuit is intended for absorbing any surge voltage produced at the time of turn-off. Current snubber circuit 71 is also intended for shunting a surge current induced by leakage inductance 15 at the time of turn-off in order to protect diode 11 from over-current damage. The circuit is made up of a resistor and a capacitor interconnected in series. The resistor dissipates part of the current energy while the capacitor smoothes the current.
In operation, when primary switch 3 is turned off, the resonance current starts flowing and voltage V.sub.DS across the primary switch begins to rise beginning with zero volts. When half a resonance cycle has elapsed and voltage V.sub.DS returns to zero volts, loop-control circuit 14 forces primary switch 3 to turn on. In this way, turn-on loss is avoided. The turn-on period is basically controlled to be proportional to the desired value of output voltage V.sub.0 of the converter and, when a deviation of output voltage V.sub.0 from the desired value is present, the turn-on period is controlled to compensate for the deviation.
FIG. 2 represents a circuit of a typical single-ended voltage-resonant forward converter provided with multiple outputs. For simplicity, a main output circuit 72 and only one auxiliary output circuit 73 are shown in the figure. The highest power output is usually selected as the main output, which is controlled by means of the voltage-resonant switching of the primary switch. In order to avoid an adverse effect exerted on the auxiliary output by the control of the main output, each auxiliary output circuit has its own output regulation circuit.
The converter provided with the voltage-resonant switching circuit and main output circuit as shown in FIG. 2, is well-known (for example, cf. Power Engineering in Electronics and Communications, Vol. 90, No. 439, PE90-68).
Auxiliary output circuit 73 is supplied with an output of secondary winding 77 of transformer 5, and rectified by rectifier diode 11. Output voltage V.sub.0 is controlled independently of main output circuit 72 by a common series dropper.
A problem encountered in the voltage-resonant DC-DC converter shown in FIG. 1, however, is that applying the resonant voltage to the primary switch inevitably involves applying a peak voltage across the switching element. Application of such a high voltage brings about voltage stress in the switching element. In order to avoid this voltage stress, it is required to use a so-called high-voltage switching element. A high-voltage switch, however, usually has high on-resistance, which causes an turn-on power loss, entailing low efficiency. Further, attaching voltage-clamping circuit 70 and current snubber circuit 71 not only brings about power losses in these circuits, but also careless use of these circuit causes the resonant voltage deform, thus allowing a turn-on loss in primary switch 3.
A problem encountered in the converter with multiple outputs is that, in order to compensate for possible variations in the input voltage of the auxiliary output circuit, it is necessary to provide in the auxiliary output circuit a series dropper having a high input voltage at the cost of a high power loss. Variations typically take place when the on-period and the switching frequency of primary switch 3 vary in response to variations of the load current or the input voltage in the main output circuit.